1. Technical Field of the Invention
The invention relates to "drilled pier" foundations. Specifically, the invention relates to the construction and installation of steel reinforcement cages in such drilled foundations and the placement of concrete therein. Most particularly, the invention discloses improved method and apparatus for the assembly and placement of such steel reinforcement cages.
2. Prior Background Art
The thump, and the thump, and the thump of the pile driver was once a familiar sound at construction sites. The sound occurs today with no where near the frequency with which it occurred in the past. More modern practice has been to drill a foundation hole into which steel reinforcement is placed and concrete poured.
As a foundation hole is being drilled, a work crew at the construction site assembles a cage of steel reinforcing bars. The resulting cage is an open cylindrical structure of steel reinforcing bars. The finished cage is disposed generally parallel to the surface of the ground above which the cage has been constructed.
The next step requires the use of a crane to lift the cage from the ground and orient it in a vertical disposition so that it can be introduced into the foundation hole which has been drilled into the earth. For relatively short reinforcing cages, the lifting of the cage is accomplished relatively easily with the assistance of workers on the ground guiding the cage as it is raised to a vertical position. The short lengths of such cages permit their lifting with minimal distortion occurring as one end of the cage is lifted while the opposite end is still bearing on the earth.
The reality of the "One hundred year flood" or the "maximum anticipated wind loading" on structures built in this fashion in the not so distant past have often shown that pier loading specifications have not been conservative enough. In addition, improvements in the art of manufacture of pre-cast concrete structural elements have increased the length of bridge spans, and the like, far beyond that anticipated in the prior art. Such factors have combined such that foundation pier loading specifications often require the on-site fabrication of steel reinforced concrete piers having diameters in excess of ten feet and axial lengths of a hundred feet or more.
When a cage of reinforcing steel is constructed on the ground and its length approaches or even exceeds one hundred feet in length, the lifting of that cage to a vertical disposition without adverse distortion becomes problematical. Despite sophisticated rigging techniques, noticeable bowing of such cages may be observed as they are raised to the vertical. The degree of such distortion may sometimes exceed the safety limit imposed on the cage structure. There is always the possibility that such excess distortion may have occurred inadvertently unnoticed by the on site inspection team.
It is an object of the present invention that the reinforcing steel cage shall be constructed in a vertical disposition such that the structure is never subjected to bending stresses as experienced in the prior art method of constructing cages horizontal to the ground.
It is a further objective of the invention that ready access to the interior of the reinforcing cage shall be provided for inspection purposes of the alignment of the cage within the earth-drilled pier hole.
It is another objective of the invention to provide for the ready, vertical alignment of the cage within the hole. Additionally, provision is made for the enhanced safety of workman employed adjacent to the reinforcing cage as it is lowered into and positioned within the earth-drilled pier hole.